The invention pertains to a method for recovering increased quantities of hydrocarbons by initially injecting steam of a high quality and then reducing the quality of the steam being injected while increasing the rate of steam injection to maintain a constant input of heat into the formation.
Numerous thermal recovery techniques have been suggested and employed to increase the recovery of hydrocarbons from underground formations. Steam flooding and water flooding have proven to be the most successful of these oil recovery techniques yet employed commercially. But after a certain percentage of oil is produced, steam flooding can be a very expensive proposition, consuming more oil for the generation of steam than is incrementally produced from the pattern. Thus, one should always focus on net oil recovery. Net oil recovery is defined as the cumulative oil produced less the cumulative oil required as fuel to generate the necessary steam.
Several methods have been developed to reduce the cost of steam flooding but yet obtain similar hydrocarbon recovery efficiencies. These methods generally involve reducing the quality of steam injected into the formation. U.S. Pat. No. 4,093,027 discloses the injection of steam having a quality between 35% and 45%. U.S. Pat. No. 4,060,129 discloses the initial injection of high quality steam followed by the injection of low quality steam or water until the cumulative value of steam quality of all of the injected fluid is reduced to a value between 35% and 45% and then continuing to inject steam having a quality between 35% and 45%. A second technique is disclosed in U.S. Pat. No. 3,360,045 wherein steam injection is followed by hot water optionally containing a polymer to increase viscosity.
Another steam injection method is disclosed in copending U.S. patent application Ser. No. 463,215, filed Feb. 2, 1983, on a steam and water injection method. This patent application pertains to a sequenced method of injecting 0.1 to 0.6 pore volumes of high quality steam, then steam of a decreasing quality down to 0% quality, followed by water at an ambient temperature. A related U.S. patent application Ser. No. 463,203, filed Feb. 2, 1983, discloses a sequenced method of injecting high quality steam, followed by steam of a decreasing quality down to 0%, followed by water and finally, in situ combustion.